Radial magnetized magnet

ABSTRACT

A permanent magnet package for electromagnetic devices, including traveling wave tubes, linear beam devices, klystrons, magnetrons, crossed field and backward wave amplifiers or oscillators and the like comprising a plurality of radially oriented linearly polarized members collectively providing a magnetic circuit. A soft iron return path between the assembled multi-segmented magnetized components completes the magnetic circuit. Magnetic materials, such as samarium-cobalt, platinumcobalt, alnico and ferrite may be utilized.

" [22] Filed:

United States Patent [191 Harrold 1 RADlAL MAGNETIZED MAGNET [75] Inventor: William J. Harrold, Littleton, Mass.

[73] Assignee: Raytheon Company, Lexington,

Mass,

June 8, 1972 [2]] Appl. No.: 260,879

[52] US. Cl. 3l5/39.71, 335/210 [51] Int. Cl. H0lj 1/00 [58] Field of Search 335/210, 284, 306; 315/35, 5.35, 39.71; 313/84 56] References Cited UNITED STATES PATENTS 3,665,242 5/1972 Dubravec 335/210 X 3,168,686 2/1965 King et al. 335/306 3,283,200 11/1966 Pallakoff 335/306 X 2,903,329 9/1959 Weber 335/302 X [111 3,781,592 Dec. 25, 1973 3,020,440 2/1962 Chang 313/84 X 3,141,116 7/1964 Henne 315/35 X 3,205,415 9/1965 Seki et al. 335/302 X Primary Examiner-George Harris Attorney-Harold A. Murphy et a1.

' 57 ABSTRACT A permanent magnet package for electromagnetic devices, including traveling wave tubes, linear beam devices, klystrons, magnetrons, crossed field and backward wave amplifiers or oscillators and the like comprising a plurality of radially oriented linearly polarized members collectively providing a magnetic circuit. A soft iron return path between the assembled multi-segmented magnetized components completes the magnetic circuit. Magnetic materials, such as Samarium-cobalt, p1atinum-cobalt, alnico and ferrite may be utilized.

5 Claims, 11 Drawing Figures PATENTEDBEEZSWS 3.781.592

SHEET 3 [1F 4 5 ALNICO xfvlzooo 1 4/ F/G. 7 8,000 m (D D --e,0o0 g 5 f I T 7" I I 0 10,000 9,000 8,000 7,000 6,000 5,000 4,000 3,000 2,000 |,O0O 0 H (OLRSTEDS) FLUX DENSITY B RESIDUAL INDUCTION SATURATION HYSTERESIS LOOP p Bm DEMAGNETIZATION\ CURVE 52 I N DU CTION COERCIVITY Y Hm H =F5OTENTIAL GRADIENT W BHC 1 RADIAL MAGNETIZED MAGNET BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to electromagnetic energy devices and, more particularly, to permanent magnets for such devices.

a 2. Description of the-Prior Art Magnetic field producing means for devices such as, for example, the magnetron as well as other devices operating in the microwaveregionof the electromagnetic spectrum commonly employ permanent magnets with the magnetic fields generally oriented parallel to the axis of the device. The term microwave is intended to define the electromagnetic energy frequency spectrum in the range above 300 MHz to 300,000 MHz or wavelengths of from one meterto one millimeter. Over the years the U-shaped, E-shaped and finally the bowlshaped permanent magnets have beenutilized for microwave devices. As higher average and peak power levels are attained permanent magnets of considerable size and weight are required to produce the high magnetic flux densities required for operation. A saturation level can readily be envisagedwhere the permanent magnet members could, weigh many hundreds of pounds which would place very stringent requirements on the utilization of microwave energy.

An excellent discussion of, the numerous applicable devices including crossed field devices as well as-linear beam devices and the utilization of permanent magnets for focusingof the electron beams inthe interaction region is found in the article fPermanent Magnets For Microwave Devices--, IEEE Transactions on Magnetics, Vol. MAG4, No. 3, Sept. 1968, pps. 229-239, by W. I-Iarrold and W. R. Reid. The development of magnetic materials having relatively high coercivity values and a high maximum energy product have succeeded in pushing the state of the art for microwave devices to higher limits for power output. Some of these materials such as platinum-cobalt which can have a cost of approximately 3500 per kilogram and other rare earth magnet materials, suchas Samarium-cobalt, have now advanced the state of the art, due to the exceedingly high energy product, to even higher potential levels. A description of the latter material is found in copending US. Pat. Application of D. K. Das, Serial No. 778,041 filed'Nov. 22, 1968, and assigned to the assignee of the present invention. Certain magnet configurations have also evolved involving a self-shielded magnetic circuit which has found application in suchdevices as the Amplitron, (a Registered Trademark of the Raytheon Company) for crossed-field amplifier tubes. With the magnetic materials reaching higher coercive force values, a problem arises in that such materials may also display lower permeance coefficient values at the. maximum energy product point. Desirably, a more linear net configuration using materials having a high coercivity is dictated. Other electromagnetic devices dependent on permanent magnets for their operation are Faraday rotators, gyrators, isolators, and multi-port circulators. Temperature sensitivity of the magnet materials 7 is a critical factor in the advancement of the power handling capabilities in such devices as well as the previously mentioned ones.

In the provision of new and improved magnetic circuits numerous factors must be considered. These factors include: weight, volume, restrictive dimensions,-

cost, required magnetizing fixtures, stray magnetic fields, external demagnetization influences, shape of the magnet and thermal environment. In numerous electromagnetic devices for airborn applications factors of weight and size are of primary importance. In the case of the high power devices, cost of the new and exotic magnet materials is to be considered. Shielding of externally mounted magnets is also desirable in numerous applications.

To further advance the state of the electromagnetic device art, therefore, new and improved permanent magnet packaging to enable the use of such new materials to evolve more efficient magnetic circuit structures is required.

SUMMARY OF THE INVENTION oriented components which have each been linearly polarized. In one embodiment the components are assembl ed in what may conceptually be considered to be a bowl magnet configuration. The magnetic circuit path provided concentrates the field density in the optimum areas to provide substantially uniformed parallel flux density lines with a companion opposing radial magnet. There is an absence, however, of the large portion of magnetic material utilized in the prior art to form the return-path. A shielding plate of soft iron is utilized at the outermost cylindrical extremity. The overall magnet package is substantially self-shielding. The invention provides for more efficient usage in terms of gap flux per kilogram of magnetic material which in the case of the costly high energy product materials can result in substantial savings. Additionally, the elimination of the bulky magnetic field return paths found in prior art U, E, and bowl-shaped pennanent magnets and the replacement of these materials by a soft iron material results in savings in weight. Internal tube components, such as pole pieces, have also been reduced in size and weight. In an illustrative embodiment a radial magnet of a very high energy product material (Samarium-cobalt) was employed in a traveling wave device with the total weight of the tube and magnet package being approximately seven pounds. The comparable prior art tube and magnet package with the same electrical parameters weighs twice this value or a total of fourteen pounds. In space application such weight savings are most important.

Other magnet configurations are illustrated including a pyramid or truncated cross section which can be utilized to provide very uniform magnetic field lines in the volume enclosed by the permanent magnets.

BRIEF DESCRIPTION OF THE DRAWINGS Details of the illustrative embodiments of the invention will be readily understood after consideration of the following description and reference to the accompanying drawings, wherein: I

FIG. 1 is a partial elevational view, partlyin cross section, of the illustrative embodiment of the invention in a crossed field device; 1

FIG. 2 is an elevational view taken along the line 2-2 in FIG. 1;

FIG. 7 is a graph illustrative of the magnetic field characteristics of several magnet materials;

FIG. 8 is a diagrammatic representation of a characteristic magnetization curve for a given magnetic material illustrating the maximum energy product;

FIG. 9 is a cross-sectional view of an alternative-radial magnetized permanent magnet package providing a substantially uniform field;

FIG. 10 is a cross-sectional view of a radial magnetized magnet package of another alternative configuration to provide more clearance for internal device components; and I FIG. 11 is a cross-sectional view of a portion of a radial magnetized magnet package for use in focussing linear beam devices.

' DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 3-6 (inclusive) some exemplary embodiments of prior art permanent magnet structures will be reviewed. In FIG. 3 a crossed field amplifier or oscillator device is provided with a cylindrical envelope 10 comprising the anode within which a centrally disposed cathode assembly (not shown) isdisposed. Pole piece members of a magnetic material are disposed on opposite sides of the envelope and the magnetic field is oriented parallel to the axis of the device indicated by the line 16. A tubular cathode support assembly 18 extends from one side of the tube envelope. A mounting plate 20 is also provided for mounting of the device in a socket. A pair of permanent magnets 22 and 24 having a rather substantial cross-sectional area is externally mounted around the tube envelope.

' In FIG. 4 the magneticfield density lines for a bowltype magnet are illustrated. The axis of symmetry designated by the line 26 would be the axis of the applicable device. An inner pole piece cylinder 28 is shown contacting a bowl magnet 30. In this magnet package configuration the bowl magnet 30 has a predetermined saturation characteristic with a residual flux density generally indicated by the art by the symbol B It is conventional in the magnet art to refer to the intrinsic induction or magnetization as41rM where M represents the number of unit dipoles per cubic centimeter.

For optimum interaction to generate or amplify magnetic energy the magnetic potential gradient lines 32 in of the pole piece member 28 are more uniformly aligned withthe axis of symmetry 26. In the prior art such materials as the alnico group have provided excellent results in crossed field devices. In some'applications the bowl magnets may be replaced by substantially U-shaped structures. In an amplifier rated at three megawatts peak and 25 kilowatts average output power a pair of U-shaped magnets capable of developing approximately 25,000 gauss weigh 18 kilograms each of an alnico material. I

Another development in the magnet art is shown in FIG. 6 where the reduced weight of the U- and E- shaped magnets are combined with the reduced flux leakage of the bowl magnets. The magnet 36 is essentially reentrant as are two of the U and F. shapes. In this embodiment an increase in the area required to carry the leakage flux in the back of the magnet is gained by increasing the width of the magnet member rather than the thickness in the section 38. The central portions of the magnet surrounding'the axial passageway 40 have a rather substantial thickness indicated in section 42.

In FIG. 5 permanent magnet field producing means for linear beam devices are shown. A plurality of magnet members 44 with intermediate pole piece members 46 providea periodic magnet focussing array useful in traveling wave devices. It will be noted that the polarity of the magnets alternates to correspond with the alternation of the energy generated by the interaction of the axial electron beam extending within the tube envelope and interaction with a periodic slow wave structure (not shown). In such devices the external magnetic field is a minute fraction of the external magnetic field surrounding an equivalent uniform field magnet of the types previously discussed. The magnetic field on the axis closely approximates a sinusoid in this embodiment. A limitation does, however,exist in such magnet configurations in that an upper limit for the period length exists beyond'which the beam is unstable and cannot be confined. Thus such devices have a limitation in the distance between the pole pieces and the coercive force is of greater significance than the maximum energy product of the magnet. The newer and exotic magnet materials, therefore, are highly attractive due to their high coercivity although many of such materials also display a low permeance coefficient at the point of maximum energy product.

Before proceeding with the discussion of the magnetron assembly of the invention, reference is directed to FIGS. 7 and 8 for a brief discussion of the magnetic characteristics of some of the representative magnetic materials as well as magnetic field behavior with respect primarily to the demagnetization parameters the region of the bowl magnettend to be nonuniform while the gradient lines 34 resulting from the directivity which determine the energy density for particular materials. In FIG. 7 some characteristic graphs of magnetic properties of several materials for use as perrnanent magnets including the new and exotic materials are shown. The demagnetization curve illustrating the residual induction and coercive force characteristics is plotted. Curve 56 illustrates the properties of platinumcobalt having a high coercivity of about 4,000 oersteds and a residual induction of about 6,000 gauss to yield an energy product of approximately 9 X 10 gauss-oersteds. This material is also referred to in the aforereferenced article and is represented to have a high cost of approximately $3500 per kilogram. This material as well as other rare earth-cobalt alloys have yielded very high performance, particularly, in airborne and spaceborne applications where the high coercivity characteristics are highly desirable.

Curve 58 is representative of one of the rare earth magnetic materials, namely a sintered composition of samarium-cobalt which can yield energy products of X 10 gauss-oersteds and higher. Such materials have extremely high coercivity characteristics but the very low incremental permeability of the demagnetization curve indicates a low permeance coefficiient is required to operate the material near its maximum energy product. This dictates the need for a thick-walled magnet return path if any of the prior art magnet configurations are utilized. One of the advantages of the new magnet materials is, however, the high coercivity which permits a much larger gap energy density for a small amount of magnet material. In addition to Samarium, other rare earth materials such as cerium, praseodymium and Ianthanurri are referred to in the referenced pending patent application. 7

Curve 60 is exemplary of the performance characteristics of the ferrite class of magnetic material which have a lower cost although the energy products are only approximately 3.5 X 10 gauss-oersteds but are nevertheless finding wide application in electromagnetic devices. 7

Curve 62 represents one of the more recently developed high coercivity members of the alnico class such as alnico-8 and -9. Another alnico class material is represented by curve 64 having a rather high residual induction such as-alnico-S.

Referring next to FIG. 8 a portion of the hysteresis loop is plotted on a B-I-I diagram and is designated the demagnetization curve 48. As is well known in the art the characteristic B indicates the flux density and H is the magnetic potential gradient or field strength. BI-Ic represents the coercivity which is the field strength required to reduce B to zero. The point 50 represents the residual induction or remanence. The symbol B, is applied to this characteristic. The line extending to the right of the B reference line represents a part of the first quadrant of the hysteresis loop which can be plotted when a sample of magnetic material is measured in a permeameter.

The energy stored in the magnetic field external to a permanently magnetized material is determined by the product of B and H and is referred to in the art as the maximum energy product expressed in terms of gaussoersteds l gauss-oersteds rim ergs per cubic centimeter). The product BH for any point P on the demagnetization curve 48 is proportional to the area of the shaded rectangle 52. The slope of a line 54 connecting point P to the origin of the plot is referred to asthe permeance coefficient for a particular material having a flux density 8,. at a potential gradient H Referring to FIGS. 1 and 2 a magnetic circuit assembly is shown for the utilization of magnetic materials having extremely high coercivity without a requirement for a large area magnetic return path. The same crossed field amplifier device is illustrated as that shown in FIG. 3 to point up the significant differences in size and weight of the respective magnet packages. The tube components as well as pole pieces have been similarly numbered as the references in FIG. 3. The embodiment of the invention comprises a first assembly of radially oriented magnet segment members 68 to collectively define a circular body. Each of the magnet members 68 form a segment of a circle with tapered sidewalls and 6 planar opposing surfaces. In exemplary embodiments for high powered crossed field amplifiers an arc represented by line of approximately 22 30 was provided for each segment. The magnet members 68 are linearly polarized as indicated by the arrows 72 with the polar designations represented by N and S. It is possible to provide eight segmented magnet members which are physically united by such means as any of the epoxy resins to form a semicircular section. Another array of eight segmented magnet members 68a are similarly secured and the two semicircular sections are joined together at the point indicated by the double line 74 by any suitable securing means. A central axial hole 76 is provided in the assembled structure to accommodate the components of a device such as pole pieces 12 and 14 of the exemplary crossed field amplifier. In the practice of the invention the overall packaged magnets are magnetized by any of the known magnetizing procedures and fixtures.

To complete the magnetic circuit assembly for the applicable device a companion opposing magnet package is provided by means of physically uniting a second assembly of radially oriented linearly polarized segmented permanent magnets 78. In this magnet package, however, the polar designations are reversed and a complete magnetic circuit is defined with the inner polar designations now N while the outer polar designations are S.

The magnetron circuit assembly including the first and second circular assemblies of the radially oriented linearly polarized segmented magnets are interconnected by an outer cylindrical shell member 80 of a ferrous composition, such as soft iron, to shield and provide a return path for the assembled magnetic field producing means. The member 80 contacts the outer extremeties of the first and second magnet packages. The magnets, particularly those of the high coercive force material, direct the required magnetic fields in the interaction regions where the crossed electric and magnetic fields produce the high electromagnetic energies. The shell member has a reduced wall thickness and lower magnetic path area ratio than the magnet members. The circuit of the present invention generates the required magnetic fields without the external leakage flux problems created by the thick-walled prior art magnet structures. In terms of gap flux per kilogram of material a much more efiicient structure is possible with extremely uniform magnetic field flux lines. The shield member 80 has considerably reduced weight and is nonmagnetized.

An exemplary embodiment of the crossed field device shown in FIGS. 1 and 2 for amplification at very high powers and frequencies in the S-band region was provided by the disclosed magnet assembly with gauss values 3700 and higher utilizing a alnico 8a material.

In FIG. 9 another embodiment of a radially oriented linearly polarized magnet package is illustrated. In linear beam applications the axial electron beam represented as a cloud of electrons 86 extends along the tube axis and between pole piece members 82 and 84. Periodic permanent magnet focussing is provided by radially oriented permanent magnet members 88 and 90 having a substantially truncated conical configuration with tapered sidewalls and united to form a circular body with an axial hole. The polar designations are reversed to provide the complete magnetic circuit. The magnetic field gradient lines are substantially uniform throughout as a result of this configuration which collectively defines a V-structure. The extra magnetic material in the wider magnet portions near the outer limits of the electron beam trajectory provides for higher gap flux densities in this region. The tapered walls 94 and 96 form the V-shape internally and the outer walls are tapered as well in this embodiment for the extra material. In the shaping and focussing of the fields for such linear beam applications an optional end shield indicated by dashed lines 99 substantially reduces external leakage flux along the back of the magnet members. The ferrous nonmagnetized return path member 98 interconnects the magnet packages and also provides shielding and return path means. The magnet may be of a segmented individually magnetized configuration or a complete body with the domains radially oriented during manufacture.

The evolution of a truncated conical design shown in FIG. 9 may be analyzed with reference being directed to FIG. 11. A segmented permanent magnet member 102 together with a pole piece member 104 and soft iron-retum path shield member 106 is shown. The substantially V-shaped wall-l08 evolves flux lines represented by the solid lines 110 and field lines represented by the dashed lines 1 12. It is noted that while the B and H fields external to the magnet coincide the internal B and H fields do not. In a permanent magnet of nonuni form cross section the tapered configuration of the type shown in the illustration has been demonstrated to provide substantially uniform magnetic fields in the focussing of linear beam devices. The distribution of the magnetic poles by the tapered magnet configuration results in the total number of dipoles being greatly increased on the outside diameter with a smaller number a of dipoles arranged around the inner diameter. The- .walls 118 and 120. Straight walls 122 and 124 extending perpendicular to the axis of the device provides additional clearance for internal tube components such as slow wave propagating structures in the vicinity of the electron beam. In this configuration the magnetic field flux lines 126 are substantially uniform in the vicinity .of the electron beam 86 while a less uniform field distribution results further away from the beam as shown by the lines 128. The additional room will also provide for clearance of such items as waveguide inputs and outputs.

The resultant radially oriented linearly polarized segmented magnet package in addition to providing more efficient magnetic fields utilizing materials of higher coercivity reduces the need for shims provided for in many of the prior art electromagnetic devices to shape the magnetic circuits.

Many other embodiments, alterations or variations will be readily apparent to those skilled in the art. It is desired, therefore, that the foregoing detailed description of the invention and the preferred embodiments be considered in the broadest aspects and not in a limiting sense.

I claim:

l. A microwave device comprising:

an envelope housing means for the generation or amplification of electromagnetic energy including an anode and a cathode;

magnetic pole piece members extending outwardly from opposing sides of said envelope;

a first circular permanent magnet package contacting one of said pole piece members comprising:

a plurality of radially oriented linear polarized segments of a magnetic material physically secured together;

a second similarly formed permanent magnet package contacting the opposing pole piece member with the polar orientation of the magnetic material dipoles in said first and second magnet packages being opposite to one another; and

means defining a shield and magnetic field return path interconnecting the outermost extremities of said first and second magnet packages.

2. A microwave device according to claim 1 wherein said shield and return path means are formed of a nonmagnetized ferrous material.

3. The device according to claim 1 wherein the magnetic field polar orientations of adjacent members in each of said first and second permanent magnet package are similar.

4. The device according to claim 1 wherein each of said magnetic segments is substantially truncated with tapered sidewalls.

5. The device according to claim 1 wherein each of saidmagnetic segments is substantially truncated and has a tapered sidewall and a substantially straight sidewall extending along a plane parallel to the envelope 

2. A microwave device according to claim 1 wherein said shield and return path means are formed of a nonmagnetized ferrous material.
 3. The device according to claim 1 wherein the magnetic field polar orientations of adjacent members in each of said first and second permanent magnet package are similar.
 4. The device according to claim 1 wherein each of said magnetic segments is substantially truncated with tapered sidewalls.
 5. The device according to claim 1 wherein each of said magnetic segments is substantially truncated and has a tapered sidewall and a substantially straight sidewall extending along a plane parallel to the envelope axis. 